An auxiliary power supply device for electric rolling stock is generally equipped with an inverter circuit for conversion of power acquired from a pantograph to AC power, a transformer for transforming voltage output from the inverter circuit, a converter circuit for conversion of the AC power output from the transformer to DC power for supply to a load, and a filter condenser for smoothing DC voltage of output from the converter circuit.
The filter condenser of the auxiliary power supply device may be discharged during maintenance, stoppage and the like of the electric rolling stock so that the filter condenser is not electrically charged at the start of running of the electric rolling stock. Under such circumstances, the filter condenser of the auxiliary power supply device is typically charged prior to the start of running.
When, during charging of the filter condenser by an auxiliary power supply device, the inverter circuit is operated in the same manner as during running of the electric rolling stock, current flowing in switching elements included in the inverter circuit is greater than the current during running of the electric rolling stock. In particular, during the time period when the amount of stored electricity of the filter condenser is nearly zero, the current in the switching elements of the inverter circuit is extremely high in comparison to when the electric rolling stock is running.
For example, Patent Literature 1 discloses technology for, during charging of the filter condenser, control of switching frequency of the switching elements by a comparison between a predetermined value of duty ratio and a triangular wave of a predetermined frequency.
The technology disclosed in Patent Literature 1 may be applied to the charging of the filter condenser of the auxiliary power supply device. For example, control can be considered so that current for charging is output from the inverter circuit in a time period T1 during which the triangular wave is less than a threshold (duty ratio), and the inverter circuit does not output current for charging during a time period T2 during which the triangular wave is greater than or equal to the threshold.
As a specific example, if the period of the triangular wave is set to 100 microseconds, and if a threshold is adopted such that the time period T1 is 3 microseconds, the time period T2 is 97 microseconds. The current output from the inverter circuit is generally a sinusoidal wave of the same period as the triangular wave, and thus if the time period T1 is 3 microseconds, the current output from the inverter circuit is cut off for a comparatively short time period. Then upon passage of 97 microseconds after blocking of the output of current, the inverter circuit again outputs current for just 3 microseconds. During the time period of charging the filter condenser of the auxiliary power supply device, such configuration enables prevention of high current flowing in the elements included in inverter circuit.